1. Field of the Invention
The field of art to which the claimed invention pertains is the fluid catalytic cracking of hydrocarbons. More specifically, the claimed invention relates to a process for the fluid catalytic cracking of hydrocarbons where a particular type of gaseous material is introduced into the reactor riser upstream of the introduction of the feed stream to be cracked in order to accomplish selective carbonization of active sites on the catalyst while accelerating the catalyst to a velocity sufficient to provide excellent catalyst-feed interaction at the point of feed introduction.
2. Background Information
There are a number of continuous cyclical processes employing fluidized solid techniques in which carbonaceous materials are deposited on the solids in the reaction zone and the solids are conveyed during the course of the cycle to another zone where carbon deposits are at least partially removed by combustion in an oxygen-containing medium. The solids from the latter zone are subsequently withdrawn and reintroduced in whole or in part to the reaction zone.
One of the more important processes of this nature is the fluid catalytic cracking (FCC) process for the conversion of relatively high-boiling hydrocarbons to lighter hydrocarbons boiling in the heating oil or gasoline (or lighter) range. The hydrocarbon feed is contacted in one or more reaction zones with the particulate cracking catalyst maintained in a fluidized state under conditions suitable for the conversion of hydrocarbons.
Obtaining the desired reaction products from a fluidized catalytic cracking zone demands proper interaction of the catalyst and the feed stream. A proper interaction will bring the catalyst having the desired activity and selectivity characteristics into contact with all feed molecules for a uniform period of time so that the most beneficial product stream is obtained without causing undue production of undesired products such as light gases and coke. Consequently, optimizing FCC operation presents chemical problems in regard to controlling activity and selectivity of the catalyst and physical problems in terms of uniformly contacting the feed with catalyst and maintaining the same period of catalyst contact for the continuously entering feed, thereby avoiding undesired side reactions such as over-cracking.
There are many references which teach, for reasons of controlling catalyst properties or catalyst flow distribution, the mixing of hot regenerated FCC catalyst with various relatively light materials prior to contact of the catalyst with the FCC feedstock. Thus, in U.S. Pat. No. 3,042,196 to Payton et al., beginning wdth a light cycle oil, progressively heavier components are added to an upflowing catalyst stream in a reactor riser so as to use a single catalyst and a single cracking zone to convert the elements of a crude oil. In U.S. Pat. No. 3,617,497 to Bryson et al. a light gas oil is mixed with a diluent vapor such as methane or ethylene at or near the bottom of a reactor riser with hot regenerated catalyst, introduced at the same point in the riser or very close downstream, with the mixture then contacted with heavy gas oil at the top of the riser so as to enhance gasoline yield. In U.S. Pat. No. 3,706,654 to Bryson et al., naphtha diluent may be added to the bottom of a reactor riser to aid in carrying upwardly into the riser the regenerated catalyst stream. In U.S. Pat. No. 3,849,291 to Owen it is disclosed that a gasiform diluent material comprising C.sub.4 + hydrocarbons and particularly C.sub.5 + hydrocarbons may be used to form a suspension with freshly regenerated catalyst which suspension is caused to flow through an initial portion of a riser reactor before bringing the hydrocarbon reactant material in contact therewith in a downstream portion of the reactor so as to achieve a very short residence time (1 to 4 seconds) that the hydrocarbon is in contact with the catalyst suspension in the riser reactor (catalyst residence time). U.S. Pat. No. 3,894,932 to Owen discusses contacting the FCC conversion catalyst with a C.sub.3 -C.sub.4 rich hydrocarbon mixture or an isobutylene rich stream before contact with gas oil boiling range feed material in an initial portion of the riser (catalyst to hydrocarbon weight ratio from 20 to 80) so as to upgrade the C.sub.3 -C.sub.4 material to a higher boiling material.
U.S. Pat. No. 4,422,925 to Williams et al. discusses passing a mixture of hydrocarbons, such as ethane, propane, butane, etc., and catalyst up through a riser reactor at an average superficial gas velocity within the range from about 40 to about 60 feet per second (12.2-18.3 meters/sec), with a catalyst to hydrocarbon weight ratio of about 5 to about 10 so as to produce normally gaseous olefins. In U.S. Pat. No. 4,427,537 to Dean et al. there is shown catalyst particles mixed with a fluidizing gas, such as a gaseous hydrocarbon, charged to a bottom portion of a reactor riser to promote or provide for a smooth non-turbulent flow up the riser of a relatively low velocity dense flow of catalyst particles.
There are additional references which show use of a lift gas in non-catalytic systems. For example, in U.S. Pat. No. 4,427,538 to Bartholic, a gas which may be a light hydrocarbon is mixed with an inert solid at the bottom part of a vertical confined conduit and a heavy petroleum fraction is introduced at a point downstream so as to vary the residence time of the petroleum fraction in the conduit. Similarly, in U.S. Pat. No. 4,427,539 to Busch et al., a C.sub.4 minus gas is used to accompany particles of little or no catalytic activity up a riser upstream of charged residual oil so as to aid in dispersing the oil.
Finally, it is taught in U.S. Pat. Nos. 4,364,848, 4,382,015, and 4,404,090 to Castillo et al. and 4,325,811 to Sorrentino that passivation of contaminating metals on an FCC catalyst may be effected by contacting hot regenerated catalyst with hydrogen and/or light hydrocarbon gas.
The process of the present invention, in contradistinction to the teachings of the above references, comprises a novel method of introducing a lift gas composition especially suited for treatment of a regenerated FCC catalyst in an FCC process in a manner that will simultaneously beneficially condition the catalyst prior to contact with feed and deliver the treated particles to the reaction zone in a flow regime which provides excellent catalyst and feed interaction.
The primary objective of the present invention is to provide an efficient method for selectively conditioning and delivering regenerated FCC catalyst to an FCC reaction zone such that the yield of desired products is maximized.